Vol. 51 No. 9 2018 713Copyright © 2018 The Society of Chemical Engineers, Japan

Journal of Chemical Engineering of Japan, Vol. 51, No. 9, pp. 713–725, 2018

Silica-Based Membranes with Molecular-Net-Sieving Properties: Development and Applications

Toshinori TsuruDepartment of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-hiroshima-shi, Hiroshima 739-8527, Japan

Keywords: Inorganic Membranes, Silica, Organosilica, Separation, Gas, Pervaporation, Reverse Osmosis

Over the past several decades, inorganic membranes composed of zeolite, silica, carbon, and metal/organic frameworks (MOF) have improved dramatically in terms of fabrication and application. Pure silica (SiO2) and organosilica membranes for use in molecular separation are the focus of this review. First, the fabrication of these silica-based membranes is out-lined to highlight the great progress achieved in sol–gel and CVD processing. Then, applications in gas- and liquid-phase separations and an evaluation of pore sizes are summarized and future perspectives are discussed.

Introduction

Polymeric membranes have a long history of intensive investigation, and they have achieved successful applica-tions that include liquid and gas-phase separation. Inorganic membranes, which are thermally and mechanically stable, have been investigated from the 1980’s, and the first Inter-national Conference on Inorganic Membranes (ICIM) was held in 1989 in Montpellier, France. The typical structures of thermally and mechanically stable inorganic membranes are summarized in Figure 1. Early-stage study on inorganic membranes in the mid-1980’s focused on membranes with large pore sizes with granular structures. Therefore, applica-tions tended toward forms of filtration such as MF and UF using Al2O3, TiO2, and ZrO2, some of which have been suc-cessfully commercialized and applied to practical applica-tions. Application of inorganic membranes to gas separation was initiated in 1980 by pioneering works on carbonized membranes. Successful separation of H2 was reported using SiO2 membranes prepared by CVD (Gavalas et al., 1989) and by sol–gel processing (Kitao et al., 1990). In addition, NaA hydrophilic and silicalite hydrophobic zeolite mem-branes with ordered structures were reported in the early 1990s. Inorganic membranes showed stability as well as high performance (high flux and selectivity), which intensified the research on inorganic membranes. Very recently, new materials such as metal organic frameworks (MOF) and carbon-based materials such as carbon nanotube (CNT) and graphene/graphene oxide have shown high performance in gas and liquid phase separation. Metal oxide, often men-tioned as ceramics such as Al2O3, SiO2, TiO2, ZrO2, and

composites, are prepared via sol–gel processing and chemi-cal vapor deposition (CVD), which includes thermal CVD and plasma-enhanced CVD (PECVD).

As shown in Figure 2, the tunable pore sizes of inor-ganic membranes are, in general, dependent on the types of materials. For example, the smallest pore size of α-Al2O3, although it is most widely used for porous ceramic mem-branes due to its high stability, is approximately 100 nm. Separation of molecules typically requires a size that is less than 1 nm, and this has been accomplished using zeo-lite, carbon, and silica-based materials. Silicon oxide (silica, SiO2) membranes have been most extensively reported due to a variety of advantages (Tsuru, 2001, 2008, 2011). Silica confers great advantages with its thermal stability, and in the controllability of pore sizes that range from subnano to several nanometers. More importantly, silica remains an amorphous structure up to 1,000°C, so that the amorphous networks are responsible for molecular separation, while

Received on July 11, 2017; accepted on August 22, 2017DOI: 10.1252/jcej.17we235Correspondence concerning this article should be addressed to T. Tsuru (E-mail address: tsuru@hiroshima-u.ac.jp).Presented at The 5th ASCON-IEEChE 2016 at Yokohama, November 2016

Journal Review

Fig. 1　Porous structures of inorganic membranes

Fig. 2　Membrane materials and pore/molecular sizes

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most metal oxides such as TiO2 and ZrO2 show granular structures and are crystallized at 400–600°C, which enlarges pore size during the phase change. Silica is rather flexible compared with other types of metal oxides, and is relatively amenable to the formation of ultramicro/nanoporous mem-branes with pore sizes of less than 1 nm, which makes it possible to separate gas mixtures such as hydrogen. On the other hand, the flexibility of silica networks causes silica membranes to suffer from densification under hydrothermal conditions, which results in decreased permeance compared with dry conditions. Organosilica, which includes hybrids of silica with organic compounds, has been extensively investi-gated almost from the beginning of the development of in-organic membranes. Since the development of organosilica using bridged alkoxysilanes, which will be introduced later, research has accelerated extensively (Agirre et al., 2014; ten Elshof and Dra, 2016).

This review is focused on silica-based membranes for molecular separation, which includes both pure silica (SiO2) and organosilica. First, fabrication of silica-based mem-branes is summarized, which highlights the great progress made in sol–gel and CVD processing. Then, applications in gas and liquid phase separation and evaluation of pore sizes will be reviewed.

1.　Fabrication of Silica Membranes

Porous separation membranes must consist of pores con-tinuously connected from the feed to the permeate stream; otherwise, no permeation through the membranes could occur. Figure 3 shows the typical structure of porous ce-ramic membranes. Supports, which are usually fabricated from metal oxide powders such as α-Al2O3 by extrusion or slip casting, show pore sizes typically larger than 1 µm and a thickness in millimeters for mechanical strength. An inter-mediate layer, mostly prepared from γ-Al2O3 or SiO2–ZrO2, is coated onto the support layer to decrease pore sizes down to several nanometers. The separation top layer, which has separation ability and must have controlled pore sizes suit-able for specific separation, is formed as thin as possible on the intermediate layer. In this way, inorganic membranes show a gradient structure that ranges from micrometers (supports) to nano/subnanometers (separation top layer) in order to minimize the resistance to permeation across a membrane.

As schematically shown in Figure 4, CVD silica mem-branes are fabricated either in (a) one-side CVD where all reactants are fed from one side for silica layer deposition, or (b) counter-diffusion CVD. Since a Si-precursor such as tetraethoxysilane (TEOS) and another reactant are fed sepa-rately from both sides of the substrate in counter-diffusion CVD, the deposition automatically terminates once a silica layer forms inside the pores, resulting in a thin layer for-mation and a reduction in pinholes. On the other hand, as shown in Figure 5, during sol–gel processing, a metal alkoxide or inorganic salt is hydrolyzed and subjected to simultaneous condensation reactions to form polymeric or

colloidal sols, depending on the reaction conditions (type of alkoxysilane and solvent, catalyst, composition of reactants, temperature, etc.). In the polymeric sol route, the hydrolysis reaction is slower, which is generally achieved by adding a small amount of water, resulting in a partially hydrolyzed alkoxide and in the formation of a linear inorganic polymer. Colloidal sols are obtained with high rates of hydrolysis and condensation. Through the subsequent gelation process, a coated layer forms on the intermediate layer. Pore sizes ef-fective for separation can be controlled by the void spaces among the packed colloidal solid (interparticle pore) in the colloidal sol route and by the gel network space in the poly-meric gel route, which corresponds to granular and amor-phous silica networks, respectively.

The most critical issues to be solved for SiO2 membranes are (1) separation performance that is affected by pore size control and defect-free thin-film formation, and (2) stability, which applies to hydrothermal stability in particular. Recent papers have focused either on one or the other of the above two issues.

Recent approaches to membrane fabrication for improv-

Fig. 3　Cross sectional SEM

Fig. 4　CVD of SiO2 membranes

Fig. 5　Sol–gel processing of SiO2 membranes

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ing the separation performance of silica-based membranes are summarized in Table 1, which can be categorized from the viewpoint of materials, structure, and processing. The advances will be introduced in the following sections, and increased focus will be applied to organosilica membranes in both CVD and sol–gel processing, which have shown great success.

1.1　Materials for the improved performance of silica-based membranes

1.1.1　Ion-doping: cations and anions　Among the vari-ous types of metal oxides, silica is rather flexible so that it is relatively easy to dope various types of ions into a silica matrix. Zr was incorporated into silica networks by co-hy-drolysis of TEOS and zirconium alkoxide such as zirconium butoxide, and successfully applied to nanofiltration, per-vaporation of aqueous solutions (Tsuru et al., 1998; Asaeda et al., 2002) and hydrogen separation under hydrothermal conditions (Yoshida et al., 2001). Ni and Co nitrates were mixed for hydrolysis and condensation of TEOS in sol–gel processing, and were reported to be effective in improving hydrothermal stability at 500–600°C under steam partial pressure of 300–400 kPa (Kanezashi and Asaeda, 2005; Igi et al., 2008). The state of doping with cations remains un-clear, but doped cations reportedly decreased the thermally induced molecular motion of a silica matrix, thereby in-creasing the hydrothermal stability. Anions such as fluorine are now being doped into silica networks in the form of ammonium fluoride. Doped fluoride, which bonds with Si atoms, reportedly expands the network structure, which controls for the production of larger pore sizes (Kanezashi et al., 2016).

Early on, CVD techniques adopted metal doping into sili-

ca; recently, Ahn et al. (2017) prepared silica–zirconia mem-branes by cofeeding TEOS and zirconium tetra-butoxide, resulting in improved hydrothermal stability.

1.1.2　Organosilica in sol–gel processing　TEOS is the most commonly used precursor for membranes. Due to the flexible control of both the chemical and physical structures of preparation parameters such as concentration, pH, reac-tion temperatures, etc., the successful tuning of the pore sizes of TEOS-derived SiO2 membranes was made possible for hydrogen separation, which requires a pore size of less than 0.4 nm, CO2 separation, and organic gas separation. Recent progress in the pore size tuning of silica-based mem-branes includes the utilization of silsesquioxane, which is an organosilicon compound with the chemical formula RSiO1.5 (R=H, alkyl, aryl or alkoxyl groups). Silsesquioxane can be categorized as a pendant type such as Si–CH3 consisting of organic groups directly bonded to Si atoms, or a bridged type such as Si–C2H4–Si units consisting of organic linking units between two silicon atoms. Sol–gel processing offers the great advantage of low-temperature processing so that organic functional groups remain following hydrolysis and condensation reactions where they can be useful in control-ling the network pore sizes and in promoting affinity with permeating molecules.

The template technique was proposed by Raman and Brinker for pore-size tuning (Raman and Brinker, 1995). As shown in Figure 6 (left), they first co-polymerized TEOS and methyltriethoxysilane (MTES), a typical pendant-type alkoxysilane, and then coated them onto an γ-Al2O3 inter-mediate layer, followed by firing under N2 to solidify the coated layer, and then firing under an air atmosphere to remove organic functional groups (methyl groups) to con-trol the pore sizes. CO2/CH4 selectivity reached 70 with CO2 permeance of approximately 10−7 mol/(m2 s Pa). These re-searchers referred to this method as a “template technique” that could be used to tune the pore sizes of silica networks. Kusakabe et al. (1999) reported silica membranes coated by TEOS mixed with octyl-, dodecyl- or octadecyltriethoxysi-lane, and found H2 and CO2 permeance was increased due to the removal of organic functional groups by calcinating this green membrane at 600°C, while the permeance of large molecules was decreased as a result of shrinkage of the silica matrix.

Another strategy involves the use of functional groups of silsesquioxane to control the hydrophilicity/hydrophobicity and selective permeation. As mentioned, silica is hydrophil-ic, and tends to suffer from decreased permeance due to ad-sorbed water. de Vos et al. (1999) prepared hydrophobic sili-

Table 1　New approaches for fabricating silica-based membranes

1.　MaterialsIon doping: doping cations such as Ni, Co, Zr, etc. for improved

hydrothermal stability, fluorine-doping for pore size control.Organosilica: a variety of organosilicon compounds such as

silsesquioxane (pendant or bridged alkoxysilane) are used for pore-size control in template or spacer methods, and for control of hydrophobicity/hydrophilicity and affinity.

Carbonized-template silica: hydrocarbon polymer was added to alkoxysilane for improved hydrothermal stability.

2.　Structural controlInterlayer-free: No intermediate layer is used for reducing

permeation resistance and facile processing.Layered hybrid: Organosilica layers are coated onto polymeric

substrates.Hydrophobic intermediate layer: to avoid any capillary con-

densation for use in humidified atmosphere.

3.　ProcessingHigh-temperature �ring: for improved hydrothermal stability.Plasma-enhanced CVD: low-pressure and atmospheric pres-

sure CVD for silica, organosilica and carbon.Interfacial polymerization: ammonium-type POSS (polyhedral

oligomeric silsesquioxane) in water and 6-FDA (hexafluoro-isopropylidene dianhydride) in toluene.

Fig. 6　Pore size control using organosilica

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ca membranes for gas separation using methylated SiO2 sols (Me-SiO2) prepared via the co-hydrolysis and condensa-tion reaction of methyltrimethoxysilane (MTMS) and TEOS using an acid catalyst. The Me-SiO2 membrane showed very high H2 permeance (1×10−5 mol/(m2 s Pa) after correcting the support resistance and high selectivity of large mole-cules. Aminopropyl triethoxysilane (APTES) was applied for CO2 selective permeation over N2 (Xomeritakis et al., 2005). The highest CO2/N2 selectivity ranged from 100–200 with CO2 permeance ranging from 1–11×10−8 mol/(m2 s Pa).

In 2008, organosilica membranes utilizing bridged-type alkoxysilanes, which consisted of linking units, mostly organic functional groups, between two Si atoms, that were first reported by a group from Twente Univer-sity, the University of Amsterdam, and the Energy Cen-ter of The Netherlands (ECN) (Castricum et al., 2008a, 2008b). Bis(triethoxysilyl) ethane (Si–C2H4–Si unit, BTESE) and bis(triethoxysilyl) methane (Si–CH2–Si unit, BTESM), showed superior n-butanol dehydration properties and hydrothermal stability compared with conventional SiO2 membranes during pervaporation at 150°C. Gas permeation properties of organosilica membranes were also reported (Kanezashi et al., 2009). The network pore size was suc-cessfully tuned by utilizing BTESE, which showed high H2 permeance with high selectivities of H2/organic compounds such as propane and toluene (Niimi et al., 2014; Yu et al., 2016), since the linking portion (Si–C2H4–Si) as a minimum unit forms loose silica networks that are larger than ≡Si–O (TEOS). This concept was proposed as a “spacer technique,” and has been confirmed by molecular dynamic simulation (Chang et al., 2010). Thus, it can be concluded that hydro-thermally stable organosilica silica, which contains a variety of carbon contents in an amorphous structure, should be a promising membrane material for the practical applica-tion of gas and liquid separation. It should be noted that the spacer technique could be combined with the template tech-nique. Lee et al. (2011) applied tetraethoxy-dimethyl disilox-ane, which consists of a siloxane linking unit and a methyl group, and successfully controlled pore sizes to within a range of 0.6–1.0 nm. Superior properties and excellent per-formance of bridged alkoxysilane makes organosilica mem-branes attractive, and has motivated extensive study.

It is difficult to draw conclusions as to general trends since each factor affects the structure of sols and gels in a complex manner. Obviously, the most crucial factor is the type of precursors that are the linking units between two Si atoms. Figure 7 shows the effect of linking units on gas permeation properties (Kanezashi et al., 2012a). With an increase in the linking unit, permeance increases and permselectivity for H2/N2 decreases, from more than 100 for TEOS to 10–20 for BTESE membranes, suggesting larger pore sizes. The effect of organic linking units, including lin-ear alkane (flexible) and aromatics (rigid) on temperature dependency, suggests that the flexible bridging units show a higher level of temperature dependence for permeation properties, which is similar to that of polymeric membranes (Castricum et al., 2011; Ren et al., 2014; Kanezashi et al.,

2017). Recently, many types of bridged alkoxysilanes have been reported, which include triazine (Ibrahim et al., 2014), pyrimidine (Yu et al., 2017), malonamide (Besselink et al., 2015), and triazole (Yamamoto et al., 2017).

Regarding the effect of sol preparation conditions, Castri-cum et al. (2015) reported that network pore sizes became dense with a decrease in acid concentration, and combined with the coating under a dried intermediate layer, H2/N2 se-lectivity increased several hundred folds. Niimi et al. (2014) reported silica networks become dense with an increase in the water ratio under a constant acid ratio. Sol size is also important in the coating process since small sol sizes allow penetration into the intermediate layer, which results in a thicker coating. A new technique to increase the size of the sol without changing the network pore size involves pH swing sols where a specific amount of NH3 is added to acid sols that switch to acid after a reaction of several minutes (Yu et al., 2016). The size of BTESE-derived sols that are created using pH-swing are controlled via the H2O/BTESE molar ratio and the reaction time in alkali. Under a H2O/BTESE ratio of 60, the BTESE-derived sols prepared using the pH-swing method show an increased sol size that con-trasts with that of the acid method, and the sol size is eas-ily controlled via the dominating reaction in alkali pH—the condensation reaction. Gas permeation results have shown that He, H2, N2, C3H8, and SF6 permeate pH-swing mem-branes at approximately twice the speed of acid-sol mem-branes. Interestingly, it is suggested that the thermal stabil-ity of BTESE membranes prepared using pH-swing sols will increase, compared with those of acid-catalyzed sols, due to a higher degree of cross-linking. Doping ions such as Al, Ag, Nb, etc. into organosilica and silica is thought to enhance permselectivity. Qi et al. (2012) reported that H2/CO2 selec-tivity was greatly increased due to the formation of a Lewis acid site, which will be introduced later.

Another approach for organosilica is the addition of hy-drocarbon into a Si precursor. Duke et al. (2006) added hexyl triethyl ammonium bromide as a cationic surfactant into silica sol, and fired the coated membranes in vacuum.

Fig. 7 Effect of precursors on gas permeances of different molecular sizes (Kanezashi et al., 2012a)

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The carbonized-template molecular-sieve silica membranes showed improved hydrothermal stability from the presence of 0.2 wt% carbon templates in the silica networks, which re-duced the movement and/or diffusion of the silanol groups and prevented them from forming densified networks.

1.1.3　Organosilica in CVD　When CVD was first re-ported by Gavalas et al. (1989), porous SiO2 membranes could then be activated for gas separation. As Nomura et al. (2005) reported reproducible production using TMOS in the counter-diffusion mode, most CVD-derived silica mem-branes have shown a high selectivity to hydrogen over other gases such as N2 and CO2. However, it is rather difficult to fabricate silica membranes with larger pore sizes. Organosil-icon compounds containing Si–C bonding in the structure were reported using phenyl groups (Ohta et al., 2008; Zhang et al., 2016) or alkyl groups (Nomura et al., 2007, 2014). Nakao group used tetramethoxysilane (TMOS), phenylmtri-methoxysilane (PTMS), diphenyldimethxysilane (DPDMS), and triphenylmethoxysilane (TPMS) in a counter diffusion technique (Ohta et al., 2008; Zhang et al., 2016). A SiO–CH3 bond in methoxide groups reportedly decomposes more easily, and the intermediate of the precursors undergoes oligomerization to form silica networks, which is followed by an oxidation reaction. The resultant pore sizes can be tuned according to the number of bulky phenyl groups. The proposed mechanism is similar to the template technique that is used in sol–gel processing. Figure 8 shows gas per-meance as a function of the kinetic diameters for TPMS-deposited membranes together with DPDMS. Following a counter-diffusion CVD time of 10 min, the size dependen-cies of gas permeance were approximate even after further CVD time, and no change in permeance was observed, indicating that CVD was terminated after formation of the CVD layer in/inside the membranes. TPMS showed a high H2 permeance of 10−6 mol/(m2 s Pa) with excellent H2/SF selectivity of 12,000 at 573 K.

1.2　Structural control for the improved performance of silica-based membranes

As shown in Figure 3, the typical structure of a silica membrane combines a silica top layer with an intermediate

layer and support layers. Multi-layered structures require firing processes for each layer, which increases cost, pro-cessing time, and requires manual handling. The primary roles of interlayers are to smooth the surface roughness to reduce defects and cracks, which helps prevent the silica sol from penetrating the porous supports. A new concept for an interlayer-free design was proposed by the da Costa group (Liu et al., 2015; Yang et al., 2017). They directly coated an α-alumina macroporous substrate with polymeric silica sols mixed with colloidal silica (70 nm in size) sols, and con-cluded that heterogeneous structures could effectively block and/or repair large pores and defects in supports without interlayers.

Typical intermediate layers are made from γ-Al2O3 or SiO2–ZrO2, both of which are hydrophilic. In practical op-erations, such as removing CO2 from flue gases or natural gas, membrane-based gas separations are processed under wet conditions. In the presence of water vapor, water can fill the pores of hydrophilic intermediate layers due to capil-lary condensation, which can be resistant to gas transport through the membranes. Hydrophobic intermediate layers of several nms, made from methyl-SiO2 sols(Me-SiO2) pre-pared via the co-hydrolysis and condensation reaction of methyltrimethoxysilane (MTMS) and TEOS using an alkali catalyst (Tsuru et al., 2011a), have been proposed and suc-cessfully applied to CO2 permeation under wet conditions (Ren et al., 2015).

A completely different approach involves the use of poly-meric nanoporous membranes for deposition of the silica top layer. Ngamou et al. (2013) successfully deposited a BTESE-derived silica layer onto porous polyamide-imide substrates via an expanding thermal plasma chemical vapor deposition (ETP-CVD) approach. Unfortunately, the carbon bridges were oxidized by the abundant reactive oxygen spe-cies due to a thermally enhanced oxidation reaction during plasma deposition. Reportedly, approximately 30% of the Si–C–C–Si bridges were retained in the BTESE networks, which resulted in the separation of pure BTESE and pure SiO2. On the other hand, Gong et al. (2014) proposed a facile sol–gel coating process where a thin and uniform BTESE-derived silica layer was successfully deposited onto a porous polysulfone support, and successfully prepared the layered hybrid membrane with a stable water flux of 2.3 kg/(m2 h) and a separation factor of approximately 2,500 for the dehydration of IPA/water (IPA: 90 wt%) solutions at 105°C during vapor permeation (VP).

1.3　Processing for the improved performance of silica-based membranes

The effect of firing temperature was examined for MTES and TEOS-derived membranes. Higher firing temperatures densified the silica networks due to the increased condensa-tion of silanol groups (de Vos et al., 1999), which increased the hydrothermal stability (Kanezashi et al., 2014). When firing organosilica at higher temperatures, densification and decomposition of organic groups occurs simultaneously, which leads to a complicated process but allows additional

Fig. 8 Gas permeance of TPMS-derived SiO2 membranes prepared for different CVD time (Zhang et al., 2016)

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control of pore-size distribution (Lee et al., 2011).Plasma-enhanced chemical vapor deposition (PECVD)

is commonly used to prepare thin films, and has been utilized in preparing silica membranes. Highly permselec-tive organosilica membranes were deposited onto porous ceramic supports using hexamethyldisiloxane (HMDSO) as a precursor via a newly developed two-step PECVD technique, which involved deposition in Ar plasma, and subsequently in O2 plasma for high performance, as shown in Figure 9 (Tsuru et al., 2011b). Recently, atmospheric-pressure PECVD (AP-PECVD) was first applied to the fab-rication of silica membranes for gas separation (Nagasawa et al., 2017). HMDSO was selected as a silicon precursor, and was fed into an afterglow region of atmospheric-pres-sure plasma to deposit a silica layer onto a porous sup-port. As-deposited membranes showed He permeance of 1.1×10−7 mol/(m2 s Pa) and a He/N2 permeance ratio of 196, and after annealed at 300°C, showed a similar performance with increased CO2 selectivity (CO2/CH4=166), which sug-gested the formation of pores that would be effective for CO2 permeation.

Interfacial polymerization of octa-ammonium POSS (polyhedral oligomeric silsesquioxane) in water and 6-FDA (hexafluoroisopropylidene dianhydride) in tolu-ene was proposed for the production of hyper-cross-linked hybrid membranes (Raaijmakers et al., 2014). The reac-tion occurred at the water/toluene interface, with the final polyPOSS-(amic acid) thicknesses of ∼0.1 µm following a reaction time of 5 min, followed by conversion of the amic acid to cyclic imide (imidization) via heat treatment at temperatures as high as 300°C. Interfacial polymerization, which is commonly used for large-scale defect-free process-ing of polyamide desalination membranes, is of great inter-est and promising for the facile production of defect-free thin-film organosilica membranes.

2.　Application

Separation membranes are usually evaluated based on (1) selectivity, (2) permeability, and, from a practical point of view, (3) chemical and thermal stability under operating

conditions over a long period, which is closely related to the lifetime of a membrane. In addition, the cost of membranes and modules is the most critical point in the replacement of conventional separation processes for practical applications. Economical simulation has suggested that increasing the level of permeance is more important than that of selectivity once a certain level of selectivity has been reached (Merkel et al., 2012; Lin et al., 2015).

In this section, promising applications of organosilica membranes are summarized in Table 2. As much as pos-sible, the information in Table 2 refers to trade-off curves, which correlate selectivity and permeance. Some of the trade-off curves are not updated, but remain useful in ex-plaining state-of-art membrane performance for specific separation systems.

2.1　Gas separationAlthough organosilica membranes have been commer-

cialized for the dehydration of organic aqueous solutions (van Veen et al., 2011; Agirre et al., 2014), gas separation also shows promise. Since typical organosilica membranes have large pores, as mentioned, hydrogen separation from large-sized molecules has potential applications. Figure 10 shows the selectivity for H2/SF6 of a BTESE-derived or-ganosilica membrane as a function of H2 permeance; SF6, 0.55 nm in size, is one of the largest gases, and is often used as a standard for pore size distribution. Organosilica membranes have clearly shown H2 permeance higher than 10−6 mol/(m2 s Pa) with H2/SF6 permeance ratios as high as 1,000, compared with zeolite and carbon membranes, prob-ably due to the thin-film formation of a smaller number of large pores such as pinholes (Kanezashi et al., 2009). Re-cently, although the primary factors affecting gas separation performance are the linking units, BTESE-derived mem-branes showed H2/N2 selectivity that ranged from 50–400 by decreasing both acid and water ratios, which reportedly en-

Fig. 9 Selectivity (H2/N2) as a function of H2 permeance for silica membranes (Tsuru et al., 2011b).

Table 2　Typical applications of silica-based membranes

Separation systems Applications

1.　Gas-H2/N2, H2/hydrocarbon -H2 recovery, purification-CO2/CH4, CO2/N2 -CH4 from bio- and natural gas,

CO2 recovery for CCS-Organic gas (C3”/C3) -Olefin/paraffin separation in

petrochemical industries

2.　Vapor/Pervaporation-H2O/alcohol (EtOH, IPA), H2O/acetic acid,

-Dehydration of organics such as alcohols forming azeotropes

-Concentration of organics -Selective permeation of organics over water

3.　Reverse osmosis-H2O/NaCl -Water purification under harsh

conditions such as high tem-peratures

4.　Membrane reactor-MCH and HI dehydrogenation -Hydrogen production from

200–300°C.

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hanced the densification of silica networks (Castricum et al., 2015). This result is clear evidence of the controllability of a wide range of pore networks in organosilica membranes.

CO2 separation is another important application, and includes CO2 removal from natural gas and flue gases. Figure 11 shows the trade-offs for CO2/N2 and CO2/CH4 for BTESE-derived organosilica and zeolite (DDR, SAPO) membranes. DDR and SAPO-34 showed high permeance ratios (>100) for CO2/CH4, while BTESE-derived mem-branes showed the highest selectivity of 90 and CO2 per-meance of 1×10−7–2.6×10−6 mol/(m2 s Pa). BTESE-derived membranes demonstrated a moderate level of selectivity and a wide range of CO2 permeance for both CO2/N2 and CO2/CH4 (Yu et al., 2016). Qi et al. (2012) reported that H2/CO2 permselectivity was largely increased due to the formation of Lewis acid sites. In addition, Nb-doped BTESE showed excellent stability in the presence of 150 kPa steam at temperatures under 200°C, as evidenced by steady H2

permeance and excellent H2/CO2 permselectivities as high as 700 during long-term stability testing for as long as 300 h. The highest selectivity of H2 over CO2 was more than 1,000. The mechanism, however, remains in question (ten Elshof and Dra, 2016). Alkylamine-silica membranes with a pendant-type structure were prepared at 673 K via CVD using 3-aminopropyltrimethoxysilane and (3-methy-aminopropyl) trimethoxysilane as primary and secondary alkylamine-silica precursors, respectively. The secondary amine precursor showed the most promising performance among the amine-based and non-amine silica membranes, although XPS revealed that N/Si ratios were 0.03 due to py-rolysis of the amine groups (Messaoud et al., 2015). Finally, a new strategy for improving CO2 selectivity was proposed by fluorine doping into SiO2 networks, although, at present, it has not yet been applied to organosilica. Fluorine, which was doped as ammonium fluoride into TEOS-derived solu-tions, reportedly tuned the pore sizes to a looser and more uniform state than without doping, which resulted in the successful preparation of fluorine-dope silica membranes with high CO2 permeance (4×10−7 mol/(m2 s Pa)) and ex-cellent CO2/CH4 selectivity (≫300) (Kanezashi et al., 2016).

Separation of olefin from paraffin is of great interest for future applications. Currently, propylene/propane is usually separated by distillation columns of more than 100 stages with a high reflux ratio, which requires a large amount of energy consumption. Various types of membranes, includ-ing polymer, carbon, zeolite, MOF, and silica, have been ex-plored for this important separation. As shown in Figure 12, a zeolitic imidazolate framework (ZIF-8) showed excellent selectivity (∼45) that exceeded the upper bounds (solid line in the figure, based on carbon membranes). On the other hand, BTESM-derived organosilica showed a higher level of permeance and moderate-to-similar selectivity. Precise control of the silica networks by the bridged alkoxysilanes resulted in the attainment of a high separation factor (Kane-zashi et al., 2012b). In addition, metal (Al, Ag) doping was examined for the possibility that it could increase the sepa-ration performance.

Fig. 10 Trade-offs for H2/SF6 for organosilica and zeolite membranes at 200°C and 25°C. (Kanezashi et al., 2009)

Fig. 11　Trade-offs for CO2/N2 and CO2/CH4. (Yu et al., 2016)

Fig. 12 C3H6/C3H8 as a function of C3H6 permeance at 22–50°C for carbon, zeolite, and organosilica membranes (Kanezashi et al., 2012b)

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2.2　Pervaporation(PV)/vapor-permeation(VP)Castricum et al. (2008a, 2008b) successfully prepared

pure BTESE membranes for the dehydration of aqueous bu-tanol solutions, and found that BTESE membranes showed a superior separation factor that reached as high as sev-eral thousands. More importantly, the BTESE membranes showed high stability at 150°C for as many as 500 d without a decline in selectivity, and with only a gradual decrease in flux (Figure 13) (van Veen et al., 2011). Table 3 sum-marizes the PV dehydration of BTESE and BTESM-de-rived membranes for water/alcohol mixtures (Agirre et al., 2014). BTESM showed higher selectivity than BTESE due to smaller pore sizes and higher selectivity for alcohols with higher carbon numbers. Both membranes showed excellent selectivity that was better than that of BTESE–MTES copo-lymerized membranes, which suggested the importance of connectivity in silica networks.

Figures 14 and 15 show the trade-offs between the sep-aration factor and the permeance of water for the de-hydration of isopropanol (Wang et al.. 2013) and acetic acid aqueous solutions (Tsuru et al., 2012) (75°C, water concentration: 10 wt%), respectively. Generally speak-ing, organosilica and pure silica membranes show a high level of permeance and moderate separation factors, while zeolite membranes show high separation factors with moderate permeance. Polymeric membranes show com-paratively low permeance and separation factors. BTESE-derived organosilica membranes show a water permeance of (2–5)×10−6 mol/(m2 s Pa) for 90 wt% AcOH, corresponding to 2.0–4.0 kg/(m2 h), and moderate separation factors that

range from 200–500. Long-term (1,800 h) immersion test-ing in 90 wt% acetic acid at room temperature confirmed the stability of organosilica membranes. On the other hand, stronger acids, such as HNO3, are known to damage BTESE membranes. Since BTESE was reported to be stable in acid, γ-Al2O3 intermediate layers could have caused the possible damage (Castricum et al., 2008c). In another PV dehydra-tion system, stable performance was reported for N-methyl pyrrolidone (NMP) aqueous solutions (van Veen et al., 2011).

The applications mentioned above focused on the dehy-dration of organic aqueous solutions based on the selective removal of water via a molecular-sieving mechanism. An-other interesting application of organosilica membranes is the selective removal of organic compounds. Paradis et al. (2013) prepared a BTESE–RTES (R=alkyl group includ-ing methyl, propyl, hexyl and decyl) membrane and found that permselectivity was changed from water-selective to butanol-selective when the carbon number increased. Araki et al. (2016) prepared hydrophobic silica membranes using a series of pendant type alkoxysilanes, including alkoxysi-lanes, ethyltrimethoxysilane (ETMS), n-propyltrimethoxysi-lane (PrTMS), isobutyltrimethoxysilane (BTMS), hexyl-trimethoxysilane (HTMS), and phenyltrimethoxysilane

Fig. 13 Long-term separation for BTESE-derived membrane for H2O/BuOH (H2O: 5 wt%) at 150°C. (van Veen et al., 2011)

Table 3 PV dehydration performance of alcohol aqueous solutions (5 wt% water)

Alcohols T [°C]Water concentration in permeate

BTESE BTESM

n-BuOH 95 99.6 99.8n-PrOH 85 99.5 99.5EtOH 70 89.2 92.2MeOH 55 18.6 55.1

Fig. 14 Trade-offs for PV dehydration of water/IPA (water: 10 wt%, 75°C) (Wang et al., 2013)

Fig. 15 Trade-offs for PV dehydration of water/acetic acid (water: 10 wt%) (Tsuru et al., 2012)

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(PhTMS), to remove a concentration of ethyl acetate from water. All of the organosilica membranes had pore sizes that ranged from 1.0–1.3 nm, and the BTMS membrane had the largest selectivity of all membranes due to the affinity be-tween the membranes and the organic compound.

PV desalination was reported by the da Costa group. In-terlayer-free hybrid carbon-silica membranes prepared from TEOS, triethoxyvinylsilane (TEVS) and non-ionic pluronic triblock copolymer (P123) showed a high water flux with high salt rejection (more than 99%) (Yang et al., 2017).

2.3　Reverse osmosis (RO)Reverse osmosis (RO) is the most commonly used tech-

nology for water purification, and is used in a variety of applications. Polyamide thin-film composite (TFC) RO membranes currently dominate the commercial market. However, polyamide TFC membranes are quite susceptible to chlorine, which is the most widely used disinfectant in water treatment for biofouling control. The organosilica membranes derived from BTESE exhibited superior mo-lecular sieving ability for neutral solutes of low molecular weight, showing molecular weight cut-offs of approximately 60–100, and a level of conventional seawater desalination that approximated that of RO membranes. More important-ly, the organosilica membranes showed not only exception-al hydrothermal stability, but also excellent stability under chlorine exposure of more than 35,000 ppm h. Figure 16 illustrates the trade-off in terms of NaCl rejection and water permeability (Ibrahim et al., 2015). Compared with conven-tional polyamide TFC membranes, water permeance values for BTESE-derived organosilica membranes are one to two orders of magnitude lower, probably because the organo-silica membranes have more rigid and hydrophobic micro-pores (Ibrahim et al., 2015), and PA membranes have a thin separation layer (∼20 nm) with high surface area ascribed to the surface ridge-and valley structure (Kong et al., 2010).

Therefore, extensive work has been dedicated to improving water permeability. Since the organosilica is hydrothermally stable, a more interesting application will include RO at high temperatures that polymeric membranes cannot typically withstand.

2.4　Membrane reactorAnother important application of organosilica mem-

branes is the use of membrane reactors for hydrogen pro-duction. A membrane reactor that combines a catalytic reac-tion and separation in one unit can shift the equilibrium and purify hydrogen as a result of the selective hydrogen extrac-tion, resulting in enhanced conversion in reactor/separation units of smaller size and/or reduced reaction temperatures. Typical reactions, as summarized in Table 4, include stream reforming methane (500–600°C, under steam), NH3 decom-position (400–500°C, without steam), and organic hydrides such as methylcyclohexane (MCH)/toluene (T) (Meng et al., 2015). Dehydrogenation of organic hydrides, which requires relatively low temperatures (250–300°C) under dry condi-tions, could make organosilica membranes more feasible than palladium-based membranes, which have shown de-creased permeance and suffer from instability at low tem-peratures. In addition, a larger difference in the molecular size between hydrogen and organic components allows mi-croporous membranes with high selectivity for hydrogen over other gases to be alternative materials for hydrogen production. MCH conversion was increased to 75%, which was higher than the equilibrium conversion of 60%, with hydrogen purity in the permeate stream of more than 99.9% at 230°C (Niimi et al., 2014; Meng et al., 2015). Membrane reactor operation longer than 1,000 h confirmed the stabil-ity of organosilica membranes CVD-derived from DMDPS (Akamatsu et al., 2015).

Another new application of a membrane reactor, hydro-gen purification during a hydrogen iodine decomposition reaction, has been proposed and examined using CVD silica membranes derived from hexylmethoxysilane (Myagmarjav et al., 2017).

3.　Transport Model

The permeation mechanism of gases through nonporous membranes is well described with the following solution-diffusion (SD) model (Adams et al., 2017)

0 0exp expp D sE E HP P PRT RT− − − ∗ ∗ ∗

= = (1)

Fig. 16 NaCl rejection and water permeability for RO desalination (Ibrahim et al., 2015)

Table 4　Hydrogen production in membrane reactors

Reaction system Temperature [°C] Dry/steamed Permeate/retentate

Steam reforming of methane 500–600 Hydro-thermal H2/CH4, CO2, CO, H2ONH3 decomposition 400–500 Dry H2/NH3, N2

Methylcyclohexane (MCH) dehydrogenation 200–300 Dry H2/ toluene, MCH

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Here, P0* is the pre-exponential factors, Ep and ED are the apparent activation energies for permeation and diffusion, respectively, and ΔHs is the effective sorption enthalpy.

On the other hand, porous membranes can be categorized as viscous flow (molecular diffusion), Knudsen, surface dif-fusion, and molecular sieving, depending on the structures (molecular size and shape, pore size), the interaction be-tween permeating molecules and membrane pore walls, and the operating conditions (pressure, temperature). For gas separation through subnano-meter levels, the modi-fied gas translation model (mGT) was recently proposed by considering the effective diffusion distance (dp–di) of the i-th component in the original GT equation. The permeance, Pi, through a membrane (pore diameter, dp; porosity, ε; tortuos-ity, τ, thickness, L) is formulated as follows (Lee et al., 2011).

2,

2

,0,

( ) 8 1ε( ) exp3

exp

p i p ii p i

ip

p ii

i

d d ERTP d d πM RTτL RTd

EkRTM RT

− − −

−

=

=

(2)

Here, Mi and Ep,i, are the molecular weight and the kinetic energy of the i-th component, respectively; R is the universal gas constant, and T is the absolute temperature.

Based on Eq. (2), normalized Knudsen-based permeance (NKP), f, that is, the permeance ratio of the i-th component to that predicted from He based on the Knudsen diffusion mechanism, can be analyzed as follows, under the assump-tion of a negligible difference in the kinetic energy of per-meation.

3

3HeHe He

(1 )f

(1 // )/i pi

pi

d dPd dP M M

= = (3)

For a more precise analysis, the mGT model was further extended by considering the activation energy of each gas component (Yoshioka et al., 2013).

The concept of NKP was verified using the permeance data of MFI and DDR zeolite membranes consisting of intrinsic pore sizes. It was applied to the quantitative evalu-ation of amorphous silica membranes (Lee et al., 2011). Figure 17 (top) shows how NKP could be applied for the analysis of pore sizes. The NKP values for each gas were best-fitted to obtain pore size, dp, of 0.34, 0.52, and 0.76 nm for TEOS, MTES and PhTES membranes, respectively. With an increase in pore size, TEOS and BTESE-derived sili-ca membranes showed increased permeance. MTES and PhTES-derived membranes showed relatively large pore sizes, but the H2 permeances were more than one order lower than those of TEOS and BTESE membranes. This was probably because the effective pore space of the silica matrix available for gas permeation was decreased due to space oc-cupation by the pendant groups (Li et al., 2011). In this way, the mGT model would be useful to quantitatively discuss the pore sizes at a subnano-meter level, along with the per-meation properties.

Another point that should be addressed is the applicabil-

ity of the two models (SD and mGT). The mGT model was originally derived under an assumption of cylindrical and rigid pores, while nonporous structures, typically a dense polymer, were assumed for the solution-diffusion model. When the pore sizes became smaller at the subnano-meter level, the difference between the two models was unclear. In fact, Koros and co-authors applied the SD model to micro-porous silica membranes prepared by pyrolyzing silanol-ter-minated PDMS/TEOS-derived films under oxygen at 377°C (Adams et al., 2017). As pointed out previously, organosilica is actually a hybrid at the molecular level with a structure that lies somewhere between a flexible polymer and a rigid siloxane. Examples of organosilica with a higher ratio of carbon/silicon (C/Si) are reported to show a higher level of activated energy for permeation (Castricum et al., 2011; Ren et al., 2014; Kanezashi et al., 2017). An understand-ing of the transport mechanism through subnano-porous membranes is integral to understanding and predicting per-meation properties such as the effects of molecular size, pore size, and temperature dependency from theoretical and practical viewpoints, all of which impact the design of high-performance membranes.

Conclusions

This review outlined the current status of silica-based membranes in terms of preparation, application and char-acterization. Recent progress in the fabrication of mem-

Fig. 17 Top: Normalized Knudsen-based curves for TEOS, MTES, PhTES membranes (curves are fitted using Eq. 2), Bottom: H2 permeance as a function of average pore sizes (Li et al., 2011)

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branes was discussed following the general categorization of materials, structural control, and processing. Regarding materials, organosilica membranes, including pendant and bridged types, have made great progress in the past decade. Organosilica membranes were successfully prepared by sol–gel processing and CVD technique, and show great potential in gas and liquid phase separation. In particular, bridged alkoxysilane, which was first reported in 2008 using bis-triethoxysilylethane (BTESE), has ushered in a new stage for silica membranes. A great variety of organosilica can be de-veloped using different types of precursors. Further study on the control and processing of silica networks, together with characterization, will facilitate future industrial applications.

Acknowledgement

This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 15H02313 and 18H03855.

Literature Cited

Adams, J., N. Bighane and W. J. Koros; “Pore Morphology and Tem-perature Dependence of Gas Transport Properties of Silica Mem-branes Derived from Oxidative Thermolysis of Polydimethylsilox-ane,” J. Membr. Sci., 524, 585–595 (2017)

Agirre, I., P. L. Arias, H. L. Castricum, M. Creatore, J. E. ten Elshof, G. G. Paradis, P. H. T. Ngamou, H. M. van Veen and J. F. Vente; “Hy-brid Organosilica Membranes and Processes: Status and Outlook,” Separ. Purif. Technol., 121, 2–12 (2014)

Ahn, S. J., A. Takagaki, T. Sugawara, R. Kikuchi and S. T. Oyama; “Permeation Properties of Silica–Zirconia Composite Membranes Supported on Porous Alumina Substrates,” J. Membr. Sci., 526, 409–416 (2017)

Akamatsu, K., T. Tago, M. Seshimo and S. Nakao; “Long-Term Stable H2 Production from Methylcyclohexane Using a Membrane Re-actor with a Dimethoxydiphenylsilane-Derived Silica Membrane Prepared via Chemical Vapor Deposition,” Ind. Eng. Chem. Res., 54, 3996–4000 (2015)

Araki, S., D. Gondo, S. Imasaka and H. Yamamoto; “Permeation Prop-erties of Organic Compounds from Aqueous Solutions through Hydrophobic Silica Membranes with Different Functional Groups by Pervaporation,” J. Membr. Sci., 514, 458–466 (2016)

Asaeda, M., J. Yang and Y. Sakou; “Porous Silica–Zirconia (50%) Mem-branes for Pervaporation of iso-Propyl Alcohol (IPA)/Water Mix-tures,” J. Chem. Eng. Japan, 35, 365–371 (2002)

Besselink, R., H. Qureshi, L. Winnubst and J. E. ten Elshof; “A Novel Malonamide Bridged Silsesquioxane Precursor for Enhanced Dis-persion of Transition Metal Ions in Hybrid Silica Membranes,” Microporous Mesoporous Mater., 214, 45–53 (2015)

Castricum, H. L., A. Sah, R. Kreiter, D. H. A. Blank, J. F. Vente and J. E. ten Elshof; “Hybrid Ceramic Nanosieves: Stabilizing Nanopores with Organic Links,” Chem. Commun. (Camb.), 9, 1103–1105 (2008a)

Castricum, H. L., A. Sah, R. Kreiter, D. H. A. Blank, J. F. Vente and J. E. ten Elshof; “Hydrothermally Stable Molecular Separation Membranes from Organically Linked Silica,” J. Mater. Chem., 18, 2150–2158 (2008b)

Castricum, H. L., R. Kreiter, H. D. van Veen, D. H. A. Blank, J. F. Vente and J. E. ten Elshof; “High-Performance Hybrid Pervaporation Membranes with Superior Hydrothermal and Acid Stability,” J. Membr. Sci., 324, 111–118 (2008c)

Castricum, H. L., G. G. Paradis, M. C. Mittelmeijer-Hazeleger, R. Kreiter, J. F. Vente and J. E. ten Elshof; “Tailoring the Separation Behavior of Hybrid Organosilica Membranes by Adjusting the Structure of the Organic Bridging Group,” Adv. Funct. Mater., 21, 2319–2329 (2011)

Castricum, H. L., H. F. Qureshi, A. Nijmeijer and L. Winnubst; “Hybrid Silica Membranes with Enhanced Hydrogen and CO2 Separation Properties,” J. Membr. Sci., 488, 121–128 (2015)

Chang, K., T. Yoshioka, M. Kanezashi, T. Tsuru and K. Tung; “A Molecular Dynamics Simulation of a Homogeneous Organic–Inorganic Hybrid Silica Membrane,” Chem. Commun. (Camb.), 46, 9140–9142 (2010)

de Vos, R. M., W. F. Maier and H. Verweij; “Hydrophobic Silica Mem-branes for Gas Separation,” J. Membr. Sci., 158, 277–288 (1999)

Duke, M. C., J. C. D. da Costa, D. D. Do, P. G. Gray and G. Q. Lu; “Hy-drothermally Robust Molecular Sieve Silica for Wet Gas Separa-tion,” Adv. Funct. Mater., 16, 1215–1220 (2006)

Gavalas, G. R., C. E. Megiris and S. W. Nam; “Deposition of H2-Permse-lective Sio2 Films,” Chem. Eng. Sci., 44, 1829–1835 (1989)

Gong, G., J. Wang, H. Nagasawa, M. Kanezashi, T. Yoshioka and T. Tsuru; “Fabrication of Hybrid Silica Separation Layer on Porous Polysulfone Support and Application of the Multi-Layer Struc-tured Composite Membrane to Vapor Permeation,” J. Membr. Sci., 464, 140–148 (2014)

Ibrahim, S. M., R. Xu, H. Nagasawa, A. Naka, J. Ohshita, T. Yosh-ioka, M. Kanezashi and T. Tsuru; “A Closer Look at the Develop-ment and Performance of Organic/Inorganic Membranes Using 2,4,6-tris[3(triethoxysilyl)-1-propoxyl]-1,3,5-triazine (TTESPT),” RSC Advances, 4, 12404–12407 (2014)

Ibrahim, S. M., H. Nagasawa, M. Kanezashi and T. Tsuru; “Robust Organosilica Membranes for High Temperature Reverse Osmosis (RO) Application: Membrane Preparation, Separation Character-istics of Solutes And Membrane Regeneration,” J. Membr. Sci., 493, 515–523 (2015)

Igi, R., T. Yoshioka, Y. H. Ikuhara, Y. Iwamoto and T. Tsuru; “Character-ization of Co-Doped Silica for Improved Hydrothermal Stability and Application to Hydrogen Separation Membranes at High Tem-peratures,” J. Am. Ceram. Soc., 91, 2975–2981 (2008)

Kanezashi, M. and M. Asaeda; “Stability of H2-Permselective Ni-Doped Silica Membranes in Steam at High Temperature,” J. Chem. Eng. Japan, 38, 908–912 (2005)

Kanezashi, M., K. Yada, T. Yoshioka and T. Tsuru; “Design of Silica Net-works for Development of Highly Permeable Hydrogen Separation Membranes with Hydrothermal Stability,” J. Am. Chem. Soc., 131, 414–415 (2009)

Kanezashi, M., M. Kawano, T. Yoshioka and T. Tsuru; “Organic–Inor-ganic Hybrid Silica Membranes with Controlled Silica Network Size for Propylene/Propane Separation,” Ind. Eng. Chem. Res., 51, 944–953 (2012a)

Kanezashi, M., W. N. Shazwani, T. Yoshioka and T. Tsuru; “Separa-tion of Propylene/Propane Binary Mixtures by bis (triethoxysilyl) Methane (BTESM)-Derived Silica Membranes Fabricated at Dif-ferent Calcination Temperatures,” J. Membr. Sci., 415-416, 478–485 (2012b)

Kanezashi, M., T. Sasaki, H. Tawarayama, H. Nagasawa, T. Yoshioka and T. Tsuru; “An Experimental and Theoretical Study On Small Gas Permeation Properties through Amorphous Silica Membrane,” J. Phys. Chem. C, 118, 20323–20331 (2014)

Kanezashi, M., T. Matsutani, T. Wakihara, H. Tawarayama, H. Naga-sawa, T. Yoshioka, T. Okubo and T. Tsuru; “Tailoring the Subnano Silica Structure via Fluorine Doping for Development of Highly Permeable CO2 Separation Membranes,” ChemNanoMat, 2, 264–

724 Journal of Chemical Engineering of Japan

267 (2016)Kanezashi, M., Y. Yoneda, H. Nagasawa, T. Tsuru, K. Yamamoto and J.

Ohshita; “Gas Permeation Properties for Organosilica Membranes with Different Si/C Ratios and Evaluation of Microporous Struc-tures,” AIChE J., 63, 4491–4498 (2017)

Kitao, S., H. Kameda and M. Asaeda; “Gas Separation by thin Porous Silica Membrane of Ultra Fine Pores at High Temperature,” MAKU (Membrane), 15, 222–227 (1990)

Kong, C., M. Kanezashi, T. Yamamoto, T. Shintani and T. Tsuru; “Con-trolled Synthesis of High Performance Polyamide Membrane with Thin Dense Layer for Water Desalination,” J. Membr. Sci., 362, 76–80 (2010)

Kusakabe, K., S. Sakamoto, T. Saie and S. Morooka; “Pore Structure of Silica Membranes Formed by a Sol–Gel Technique Using Tetrae-thoxysilane and Alkyltriethoxysilanes,” Separ. Purif. Technol., 16, 139–146 (1999)

Lee, H. R., M. Kanezashi, Y. Shimomura, T. Yoshioka and T. Tsuru; “Evaluation and Fabrication of Pore-Size-Tuned Silica Membranes with Tetraethoxydimethyl Disiloxane For Gas Separation,” AIChE J., 57, 2755–2765 (2011)

Li, G., M. Kanezashi and T. Tsuru; “Preparation of Organic–Inorganic Hybrid Silica Membranes Using Organoalkoxysilanes: Effect of Pendant Groups,” J. Membr. Sci., 379, 287–295 (2011)

Lin, H., Z. He, Z. Sun, J. Kniep, A. Ng, R. W. Baker and T. C. Merkel; “CO2-Selective Membranes for Hydrogen Production and CO2 Capture—Part II: Techno-Economic Analysis,” J. Membr. Sci., 493, 794–806 (2015)

Liu, L., D. K. Wang, D. L. Martens, S. Smart and J. C. Diniz da Costa; “Interlayer-Free Microporous Cobalt Oxide Silica Membranes via Silica Seeding Sol–Gel Technique,” J. Membr. Sci., 492, 1–8 (2015)

Meng, L., X. Yu, T. Niimi, H. Nagasawa, M. Kanezashi, T. Yoshioka and T. Tsuru; “Methylcyclohexane Dehydrogenation for Hydrogen Production via a Bimodal Catalytic Membrane Reactor,” AIChE J., 61, 1628–1638 (2015)

Merkel, T. C., M. Zhou and R. W. Baker; “Carbon Dioxide Capture with Membranes at an IGCC Power Plant,” J. Membr. Sci., 389, 441–450 (2012)

Messaoud, S. B., A. Takagaki, T. Sugawara, R. Kikuchi and S. T. Oyama; “Alkylamine–Silica Hybrid Membranes for Carbon Dioxide/Meth-ane Separation,” J. Membr. Sci., 477, 161–171 (2015)

Myagmarjav, O., A. Ikeda, N. Tanaka, S. Kubo and M. Nomura; “Prepa-ration of an H2-Permselective Silica Membrane for the Separation of H2 from the Hydrogen Iodide Decomposition Reaction in the Iodine–Sulfur Process,” Int. J. Hydrogen Energy, 42, 6012–6023 (2017)

Nagasawa, H., Y. Yamamoto, N. Tsuda, M. Kanezashi, T. Yoshioka and T. Tsuru; “Atmospheric-Pressure Plasma-Enhanced Chemical Vapor Deposition of Microporous Silica Membranes for Gas Sepa-ration,” J. Membr. Sci., 524, 644–651 (2017)

Ngamou, P. H. T., J. P. Overbeek, R. Kreiter, H. M. van Veen, J. F. Vente, I. M. Wienk, P. Cuperus and M. Creatore; “Plasma-Deposited Hybrid Silica Membranes with a Controlled Retention of Organic Bridges,” J. Mater. Chem. A Mater. Energy Sustain., 1, 5567–5576 (2013)

Niimi, T., H. Nagasawa, M. Kanezashi, T. Yoshioka, K. Ito and T. Tsuru; “Preparation of BTESE-Derived Organosilica Membranes for Cat-alytic Membrane Reactors of Methylcyclohexane Dehydrogena-tion,” J. Membr. Sci., 455, 375–383 (2014)

Nomura, M., K. Ono, S. Gopalakrishnan, T. Sugawara and S. Nakao; “Preparation of a Stable Silica Membrane by a Counter Diffusion Chemical Vapor Deposition Method,” J. Membr. Sci., 251, 151–158 (2005)

Nomura, M., T. Nagayo and K. Monma; “Pore Size Control of a Molec-ular Sieve Silica Membrane Prepared by a Counter Diffusion CVD Method,” J. Chem. Eng. Japan, 40, 1235–1241 (2007)

Nomura, M., E. Matsuyama, A. Ikeda, M. Komatsuzaki and M. Sasaki; “Preparation of Silica Hybrid Membranes for High Temperature CO2 Separation,” J. Chem. Eng. Japan, 47, 569–573 (2014)

Ohta, Y., K. Akamatsu, T. Sugawara, A. Nakao, A. Miyoshi and S.-I. Nakao; “Development of Pore Size-Controlled Silica Membranes for Gas Separation by Chemical Vapor Deposition,” J. Membr. Sci., 315, 93–99 (2008)

Paradis, G. G., D. P. Shanahan and R. Kreiter; “H. M. van Veen H. L. Castricum, A. Nijmeijer and J. F. Vente; “From Hydrophilic to Hydrophobic Hybsi® Membranes: a Change of Affinity And Ap-plicability,” J. Membr. Sci., 428, 157–162 (2013)

Qi, H., H. Chen, L. Li, G. Zhu and N. Xu; “Effect of Nb Content on Hy-drothermal Stability of a Novel Ethylene-Bridged Silsesquioxane Molecular Sieving Membrane for H2/CO2 Separation,” J. Membr. Sci., 421–422, 190–200 (2012)

Raaijmakers, M. J. T., M. A. Hempenius, P. M. Schön, G. J. Vancso, A. Nijmeijer, M. Wessling and N. E. Benes; “Sieving of Hot Gases by Hyper-Cross-Linked Nanoscale-Hybrid Membranes,” J. Am. Chem. Soc., 136, 330–335 (2014)

Raman, N. K. and C. J. Brinker; “Organic “Template” Approach to Molecular Sieving Silica Membranes,” J. Membr. Sci., 105, 273–279 (1995)

Ren, X., K. Nishimoto, M. Kanezashi, H. Nagasawa, T. Yoshioka and T. Tsuru; “CO2 Permeation through Organic–Inorganic Hybrid Silica Membranes in the Presence of Water Vapor,” Ind. Eng. Chem. Res., 53, 6113–6120 (2014)

Ren, X., M. Kanezashi, H. Nagasawa and T. Tsuru; “Plasma-Assisted Multi-Layered Coating towards Improved Gas Permeation Proper-ties for Organosilica Membranes,” RSC Advances, 5, 59837–59844 (2015)

ten Elshof, J. E. and A. P. Dral; “Structure–Property Tuning in Hydro-thermally Stable Sol–Gel Processed Hybrid Organosilica Molecu-lar Sieving Membranes,” J. Sol–gel Sci. Technol., 79, 279–294 (2016)

Tsuru, T.; “Inorganic Porous Membranes for Liquid Phase Separation,” Separ. Purif. Methods, 30, 191–220 (2001)

Tsuru, T.; “Nano/Subnano-Tuning of Porous Ceramic Membranes for Molecular Separation,” J. Sol–gel Sci. Technol., 46, 349–361 (2008)

Tsuru, T.; “Development of Metal-Doped Silica Membranes for In-creased Hydrothermal Stability and their Applications to Mem-brane Reaction of Steam Reforming of Methane,” J. Jpn. Petrol. Inst., 54, 277–286 (2011)

Tsuru, T., H. Takezoe and M. Asaeda; “Ion Separation by Porous Silica-Zirconia Nanofiltration Membranes,” AIChE J., 44, 765–768 (1998)

Tsuru, T., T. Nakasuji, M. Oka, M. Kanezashi and T. Yoshioka; “Prepara-tion of Hydrophobic Nanoporous Methylated SiO2 Membranes and Application to Nanofiltration of Hexane Solutions,” J. Membr. Sci., 384, 149–156 (2011a)

Tsuru, T., H. Shigemoto, M. Kanezashi and T. Yoshioka; “2-Step Plas-ma-Enhanced CVD for Low-Temperature Fabrication of Silica Membranes with High Gas-Separation Performance,” Chem. Com-mun. (Camb.), 47, 8070–8072 (2011b)

Tsuru, T., T. Shibata, J. Wang, H. R. Lee, M. Kanezashi and T. Yoshioka; “Pervaporation of Acetic Acid Aqueous Solutions by Organosilica Membranes,” J. Membr. Sci., 421-422, 25–31 (2012)

van Veen, H. M., M. D. A. Rietkerk, D. P. Shanahan, M. M. A. van Tuel, R. Kreiter, H. L. Castricum, J. E. ten Elshof and J. F. Vente; “Push-ing Membrane Stability Boundaries with HybSiR Pervaporation Membranes,” J. Membr. Sci., 380, 124–131 (2011)

Wang, J., G. Gong, M. Kanezashi, T. Yoshioka, K. Ito and T. Tsuru;

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“Pervaporation Performance and Characterization of Organosilica Membranes with Tuned Pore Size by Solid-Phase HCl Post-Treat-ment,” J. Membr. Sci., 441, 120–128 (2013)

Xomeritakis, G., C. Y. Tsai and C. J. Brinker; “Microporous Sol–Gel Derived Aminosilicate Membrane for Enhanced Carbon Dioxide Separation,” Separ. Purif. Technol., 42, 249–257 (2005)

Yamamoto, K., M. Kanezashi, T. Tsuru and J. Ohshita; “Preparation of Bridged Polysilsesquioxane-Based Membranes Containing 1,2,3-Triazole Moieties for Water Desalination,” Polym. J., 49, 401–406 (2017)

Yang, H., M. Elma, D. K. Wang, J. Motuzas and J. C. Diniz da Costa; “Interlayer-Free Hybrid Carbon-Silica Membranes for Processing Brackish to Brine Salt Solutions by Pervaporation,” J. Membr. Sci., 523, 197–204 (2017)

Yoshida, K., Y. Hirano, H. Fujii, T. Tsuru and M. Asaeda; “Hydrother-mal Stability and Performance of Silica–Zirconia Membranes for Hydrogen Separation in Hydrothermal Conditions,” J. Chem. Eng.

Japan, 34, 523–530 (2001)Yoshioka, T., M. Kanezashi and T. Tsuru; “Micropore size Estimation on

Gas Separation Membranes: A study in Experimental and Molecu-lar Dynamics,” AIChE J., 59, 2179–2194 (2013)

Yu, L., M. Kanezashi, H. Nagasawa, J. Oshita, A. Naka and T. Tsuru; “Fabrication and Microstructure Tuning of a Pyrimidine-bridged Organoalkoxysilane Membrane for CO2 Separation,” Ind. Eng. Chem. Res., 56, 1316–1326 (2017)

Yu, X., L. Meng, T. Niimi, H. Nagasawa, M. Kanezashi, T. Yoshioka and T. Tsuru; “Network Engineering of a BTESE Membrane for Im-proved Gas Performance via a Novel Ph-Swing Method,” J. Membr. Sci., 511, 219–227 (2016)

Zhang, X. L., H. Yamada, T. Saito, T. Kai, K. Murakami, M. Nakashi-ma, J. Ohshita, K. Akamatsu and S. Nakao; “Development of Hydrogen-Selective Triphenylmethoxysilane-Derived Silica Mem-branes with Tailored Pore Size by Chemical Vapor Deposition,” J. Membr. Sci., 499, 28–35 (2016)